Activated sludge is a biomass of organisms associated with organic matter used in biological treatment of wastewater. Organic matter is the main component of the wastewater, therefore, this thesis analyzes the effect of microfauna in organic load reduction due to size of particles of the activated sludge. The structure of microfauna community in different concentration ratios of disintegrated and normal active sludge was analyzed in laboratory during the period of 72 h. The impact of microfauna on decreasing turbidity and total suspended solids was significant in all concentracion ratios. The most abundant group were ciliates and the dominant species was Vorticella convallaria. Smaller particles of activated sludge have significantly contributed to feeding and increasing growth rate of microfauna. The most abundant according to life forms were attached ciliates and according to the feeding guilds the most abundant were filtrators. Results of this study suggest that for the property functioning of wastewater treatment plant is necessary to establish a community of attached filtrator
ciliates and rotifers that contribute the most in decreasing of suspended organic matters.